The present invention comprises a new and distinct cultivar of hybrid pistachio (Pistacia atlantica (not patented) x Pistacia integerrima (not patented)) used as a rootstock known by the varietal name ‘UCB1-D15.’ The variety ‘UCB1-D15,’ is an in vitro, single selection of the segregating controlled cross of (P. atlantica x P. integerrima) made at Davis, Calif. The purpose of the selection program was to develop rootstocks that when grafted with common varieties produced superior vigor and crop yields. The present variety exhibits greater vigor and productivity than ‘UCB1-D1’ (not patented) rootstock. The present variety has higher resistance to Verticillium disease than the female parent, Pistacia atlantica. The present variety has higher cold tolerance than the male parent, Pistacia integerrima. 
The following characteristics distinguish clone ‘UCB1-D15’ from the industry standard, ‘UCB1-D1’ (not patented) rootstock:                1. Greater rootstock growth.        2. Greater scion growth.        3. Superior crop yield.        4. The present variety differs from the industry standard ‘UCB1-D1’ (not patented) rootstock in that it has consistently produced an average yield per acre that is more than double that of ‘UCB1-D1’, even in non-bearing years. The present variety differs from rootstock ‘UCB1-D11’ in that the present variety has a slightly smaller rootstock circumference, and produces a slightly smaller average yield per acre than the ‘UCB1-D11’.        